During the 5 years of the project we have made a tremendous progress in understanding recent and remote memory and the role of astrocytes in behavior. We have established the experimental systems essential for implementation of the project, and completed all 3 objectives outlined in the grant proposal. Our results were published in the best journals.
The major efforts and accomplishments are the following:
A. The parts of the project investigating the role of astrocytes in projection-specific effects on memory acquisition and the role of the CA1 and ACC projection in remote memory acquisition were completed. By employing connectivity-based tagging, to label specific populations of hippocampal based on their projection target, we have causally demonstrated the functional significance of specific projections to memory acquisition, and defined a precise time-window for this process. This objective is published in Kol et al., Nature Neuroscience 2020, and is also a part of Refaeli et al, Current Biology, 2023.
B. Our 2-photon microscope was upgraded (by ERC funding) to allow parallel 2-channel imaging of astrocytes and neurons. We have imaged astrocytic activity in mice navigating a virtual environment (Aim 3C), and made very exciting discoveries: whereas single astrocytes do not encode specific locations (as opposed to neurons), it is possible to design an encoder that will infer the location of the mouse based on the activity of the astrocytic population. Furthermore, there is a clear difference in astrocytic activity when mice are navigating a familiar or a novel environment. The work, describing how hippocampal astrocytes encode reward location was published in Nature (Doron et al, 2022).
C. Objective 1, revolve around ensembles, their activity and connectivity. We have employed activity-based fluorescent tagging to label recent and remote memory recall ensembles. To determine how they differ in their activity we checked the overlap of these populations between acquisition, recent, and remote. We found that the recent and remote populations are relatively stable, and the recent engram is necessary for remote recall. To look for changes in connectivity we employed CLARITY-based detection of the output and input projections from/to the hippocampus in a single-cell resolution. We hypothesize that remote recall ensemble cells in CA1 will be preferentially connected to frontal regions, and indeed, this is what we found (Refaeli et al, Current Biology, 2023).
D. We made a 3D, CLARITY based, spatial description of neurons and astrocytes in the hippocampus, as a basis for our future investigations in this aim. This paper about astrocyte morphology is published, Refaeli et al, Glia, 2021, accompanied by a method paper Refaeli et al, JoVE, 2022.
